1. Field of the Invention
The present invention relates to a flexible printed-circuit base board of a copper-foil/polyimide-layer/copper-foil structure excellent in dimensional stability, heat resistance and electrical characteristics.
2. Prior Art
Conventional polyimide type flexible printed-circuit base boards with metallic foils on both sides thereof are predominantly of a structure comprising metallic foils laminated on both sides of a polyimide film with an adhesive therebetween.
Since such base boards are produced using an adhesive between a polyimide film and metallic foils, however, the characteristics of the base boards depend on the properties of the adhesive and hence involve poor heat resistance, poor chemical resistance, poor moisture resistance, poor flexibility, etc., which are all very problematic.
Proposed solutions to the foregoing problems include processes wherein two single-sided copper-clad base boards comprised of a polyimide layer and a copper foil were laminated on each other with an adhesive therebetween (see Japanese Patent Laid-Open Nos. 153,393/1980 and 18,294/1989). However, these processes also involve substantially the same problems as mentioned above because of the use of an adhesive.
Further proposed solutions to these problems include improved processes wherein a polyimide film is bonded directly to copper foils under heat and pressure conditions (see Japanese Patent Laid-Open No.181,857/1982 and Japanese Patent Publication No. 15,825/1986).
These improved processes require the same number of steps as or a larger number of steps than those of the processes previously proposed because of the additional step of producing a polyimide resin film, though base boards produced thereby are improved in heat resistance and electrical characteristics since no adhesive is used.
Furthermore, these improved processes involve a problem that conspicuous curling of a base board produced thereby occurs when part of a copper foil on one side thereof is removed by etching, because there is a large difference in thermal expansion coefficient between the copper foils and the polyimide resin film of the base board, which difference is attributed to the flexible chemical structure of the molecular chains of the polyimide designed with the aim of improving the adhesion of the polyimide film to the copper foils. If use is made of a polyimide resin having a thermal expansion coefficient substantially in agreement with that of copper foils, there arises a general problem that the adhesion of the polyimide resin to copper foils tends to be low.
As a result of intensive investigations with a view to solving these problems, the inventors of the present invention have found out that a flexible double-sided copper-clad printed-circuit base board of a copper-foil/polyimide-layer/copper-foil structure excellent in dimensional stability, heat resistance and electrical characteristics, which board is endowed with a very high adhesion between the copper foils and the polyimde layer thereof and does not undergo curling even when part of a copper foil on one side of the base board is removed by ething, can be obtained by applying a solution of polyamic acids as specified in the present invention on a surface of a copper foil, curing the resulting coating film through heating to form a copper-foil/polyimide laminate, and contact-bonding another copper foil to the polyimide surface of the laminate under heat and pressure conditions. The present invention has been completed based on this finding.